The longest escalators in the Eastern hemisphere, at Wheaton Metro, which I've ridden many times, are apparently the same speed according to Wikipedia. So my gut feeling of maybe they did that because maybe they're short doesn't hold

LA: 0.46 meters/s (1 mph)

Hong Kong: 0.75 meters/s (1.7 mph)

Kyiv: 0.9 meters/s (2 mph)

Kinda slower than I expected in general. I assume the speed is mostly dictated by boarding/alighting safety.